The invention relates to a protective device for a power-assisted steering system, such a power-assisted steering system, and a method for protecting a power-assisted steering system.
With power-assisted steering systems there can be problems when the wheels move at high speeds. Thus for instance in a vehicle workshop the steering is frequently moved by rotating the wheels of the motor vehicle while the power-assisted steering is turned off. When enough force is exerted, the steering can be accelerated such that a very high steering speed is attained. This is far greater than the steering speed that occurs during normal operation. The parts of the steering system that are operationally connected to the wheels thus obtain high rotational speeds.
If the steering system stops at the steering stop at a high steering speed, all components are sharply braked. Due to the great delay in the rack and the electric motor drivingly connected thereto there can be very high torques and forces on the transmission and electric motor. These torques and forces can be so great that the components are damaged.
The higher the speed with which the stop is reached, the higher the risk of damage. Although the high rotational speeds can be avoided during operation using suitable control of the motor in that active intervention is used to prevent high rotational speeds, this is not possible when it is turned off. When it is turned off damage is probable if no other measures are taken.
It should furthermore be kept in mind that in steering systems with electrical neutral point separation using field effect transistors and a synchronous motor, at a high rotational speed the motor can generate so much voltage that the field effect transistors of the neutral point go into avalanche mode. Then a majority of the energy added by the mechanic is converted to heat in the neutral point field effect transistors. This can lead to the field effect transistors being thermally destroyed.
A protective device for an electrical steering system is known from publication DE 100 13 133 A1. In it, the electric motor for the power-assisted steering system is connected to a brake circuit that applies an electrical load to the motor if the rotational speed exceeds a prespecified value.
It is disadvantageous in the protective device described in the publication that the protective device functions only with direct current motors. However, as a rule permanently excited synchronous motors are used in electric steering systems with greater power. For safety reasons it is provided that the neutral point is separated when turned off.
Completing the circuit of the output stage, as is suggested in the publication, does not have any effect in a relay separation because the separated neutral point does not provide any current and thus does not permit braking of the motor.
During normal operations this device must be turned off because otherwise the system does not function (for instance cut-out relay). This cut-out device can itself be faulty, which reduces the availability of the steering system.
It is therefore the object to suggest a protective device that can also be used with a synchronous motor with neutral point separation.